The Stonehenge Hidden Landscape Project plans to map features hidden under the surface in an area totaling 14 square kilometers (8.7 sq miles) around the famous monument. The mysterious feature was found only two weeks into the three-year survey.

The team picked up traces of postholes, where heavy wooden poles had once been sunk into the earth. The soil in these holes is of a different density than the undisturbed soil around them and show up on the subsurface imaging. The ring of posts appears to have had two openings opposite one another and was encircled by a ring of pits a meter wide. Archaeologists say it was built about 2,500 BC, about the same time that the builders of Stonehenge switched from using timber to using stone.

Project leader Professor Vince Gaffney of the University of Birmingham said, “When you see that as an archaeologist, you just look at it and think, ‘that’s a henge monument’ – it’s a timber equivalent to Stonehenge. The monument is one of the most studied monuments on Earth but this demonstrates that there is still much more to be found. The presumption was this was just an empty field – now you’ve got a major ceremonial monument looking at Stonehenge”.

The BBC has an interview with Prof. Gaffney and a computer reconstruction of the monument here. His team’s discovery comes just weeks after the start of excavations at Marden Henge, a stone circle ten times bigger than Stonehenge. It’s shaping up to be a good summer for archaeologists!